1. Field of the Invention
The present invention relates to a CDMA communication technique and particularly, to a SIR (Signal to Interference Ratio) measuring apparatus and a SIR measuring method.
2. Description of the Related Art
In spectrum-spreading radio communications using CDMA (Code Division Multiple Access) communication system, at a transmission side, after a primary modulation is executed on data, a secondary modulation using spread codes (Pseudo Noise: PN) is executed to spread the signal band of the data and then the data thus spread are transmitted. In this case, the spread codes (pseudo noises) used for spreading are allocated while they are different among users or information channels, so that multiplexing is performed and the frequency band can be used on plural channels at the same time. At a reception side, a reception signal (data) in a broad band are subjected to de-spreading using spread codes (pseudo noises) so as to be returned to the original signal in a narrow band, and then demodulation processing is executed on the original signal thus returned.
In the de-spreading process described above, the correlation with the reception signal is executed by using the same spread codes as the spread codes (pseudo noises) used at the transmission side, and correlation detection at a desired channel is carried out by using a de-spreading filter, so that a signal desired by a user can be detected from the multiplexed reception signal.
Since cross-correlation generally exists among spreading codes, the cross-correlation is also outputted while it is contained in a de-spread output signal, and thus it generates an interference signal. If the number of users to be multiplexed increases, the number of interference signals (the magnitude of the interference signal) increases, resulting in degradation in reception characteristic. In the CDMA system for spreading signals to signals in a broader band by using higher-speed spreading codes than the period of information (symbols) and carrying out multiplexing access transmission, a pilot signal whose pattern has been known is periodically inserted between information signals to construct frames.
In a mobile communication system based on the CDMA system, transmission power is required to be optimally controlled on the basis of reception SIR (Signal to Interference Ratio) in order to reduce interference signals. In order to satisfy this requirement, it is important to estimate SIR of reception signals with high precision. SIR of a CDMA receiver is disclosed in Japanese Laid-open Patent Publication No. 2000-49662, and Japanese Laid-open Patent Publication No. 2000-68981.
Mobile communications are used under multi-path environments, and RAKE combining based on a path diversity system is generally used in the CDMA communications. As a CDMA receiver is used a RAKE receiver in which plural reception signals passing through different paths (transmission paths) are respectively subjected to de-spreading (a correlator for performing de-spreading is called as “RAKE finger”) using the reception delay amounts corresponding to the respective paths, respectively and then RAKE combining is carried out.
In this case, it is required to estimate SIR after RAKE combining. As well known, according to the RAKE combining system, a reception signal is subjected to de-spreading on the basis of path delay information detected by a path detector (path search portion), then a de-spread signal of each path is phase-detected, and then they are weighted and added so that SIR after combining is maximum.
FIG. 1 is a diagram showing an example of the construction of a conventional SIR measuring apparatus for estimating SIR after RAKE combining. Referring to FIG. 1, path detector 10 to which reception signal 1 is inputted includes plural correlators for taking the correlation between the reception signal and each of spreading codes (pseudo noise signals) that are delayed by the respective different delay amounts within a path search range. Thereafter, delay profiles are created on the basis of the correlation output values from the plural correlators, and then the delay profiles are compared with a predetermined threshold value, so that path detection is performed and path delay amount signal is outputted to de-spreaders 111 to 11k. The de-spreaders 111 to 11k take the correlation between the inputted reception signal 1 and the spreading code. In the de-spreaders 111 to 11k, the spreading code from a spreading code generator is delayed on the basis of the path delay amount signal from the path detector 10.
SIR estimators 121 to 12K receive the outputs of the de-spreaders 111 to 11k to estimate SIR. That is, by using pilot symbols, for which a transmission sequence is known, of signals after synchronous detection, the data after the detection are subjected to inverse modulation based on the known transmission sequence, and then the average value of the power of the signal thus inversely-modulated is divided by the dispersion of the inversely-modulated signal to determine SIR. That is, the average of the symbols of the known inversely-modulated pilot signal is determined, and the power Ps of the signal is calculated from the square of the amplitude of the average value. On the basis of the inversely-modulated signal and the average value, the dispersion PI of the power is determined, and the SIR estimated value R is determined from the ratio PS/PI.
In combiner 18, representing the SIR value of each path by Ri, the total of the SIR value represented by the following equation (1) is outputted.
                              ∑                      i            =            1                    K                ⁢                                  ⁢        Ri                            (        1        )            That is, the total of the SIR output 2 after RAKE combining is given by the above equation (1) where the SIR value of each path is represented by Ri (i=1 to K).
In the SIR estimation described above, the SIR value Ri of each path is first estimated, and these SIR values are added with one another to implement the SIR estimation. The output of the combiner 18 shown in FIG. 1 is supplied to a transmission power controller or the like, and used to perform the transmission power control based on the value of SIR.
Under an actual transmission environment, there may be assumed such a situation that the delay amount of each path is extremely small. Therefore, the reception characteristic is enhanced by detecting path information at a narrow path interval less than 1 chip in the path detector.
Therefore, actually, there is such a case that even when there is only one path, plural path delay amounts are detected for the one path. That is, in the case where the path interval is set at a half chip, there is such a case that two path delays are detected for one chip.
In such a case, according to the conventional SIR estimation system described above, SIR for the same path is added twice when the difference of the delay amount of the path is small, and thus a larger value than actual SIR is estimated. That is, when the apparatus is designed so that path information is detected at a path interval less than one chip in order to enhance the reception characteristic, there may occur such a case that SIR for the same path is added at plural times, so that the measurement of SIR is inaccurate, and accurate power control cannot be performed.